Abstract

Lithium niobate (LN) is an attractive material for high-Q whispering gallery mode (WGM) resonators due to its wide optical transparency window, high electro-optic coefficient and nonlinearity. Recently, LN structures suitable for WGM micro-resonators have been fabricated using surface tension reshaping of a previously micro-structured substrates, producing ultra-smooth surfaces while also maintaining the useful crystalline properties of the original material [1]. The method is based on the preferential melting of a surface layer [2] at temperatures close to the melting point for the bulk material. Upon cooling, the melted surface layer re-crystallizes, seeded by the bulk crystal that remains solid during the process, and is reshaped by the surface tension to form ultra-smooth single crystal superstructures. Fig. 1 shows some surface tension reshaped structures which have been obtained by varying the initial microstructure and the thermal treatment conditions, (a): 5µm prolate spheroid, (b): 3µm capsule, (c-e): 7, 30, 80µm diameter pillars. Such structures are suitable for supporting WGMs as the smooth side surface should lead to low scattering loss. Their obtainable small dimensions leads to a small number of supported WGMs and increases the free spectral range (FSR). Both are beneficial for the fabrication of spectral filters and lasers.

© 2011 Optical Society of America